Carbon dioxide is released into the atmosphere in the following ways:
i) Through the respiration performed by plants and animals. This involves the breaking down of glucose (or other organic molecules) into carbon dioxide and water.
ii) Through the decay of animal and plant matter. Fungi and bacteria break down the carbon compounds in dead animals and plants and convert the carbon to carbon dioxide if oxygen is present, or methane if oxygen is absent.
iii) Through combustion of organic material which oxidizes the carbon it contains, producing carbon dioxide (and other things, like water vapor). Burning fossil fuels releases carbon that has been stored in the geosphere for millions of years. Burning agrofuels also releases carbon dioxide.
iv) Carbon dioxide is released when limestone (calcium carbonate) is heated to produce lime (calcium oxide), a component of cement.
v) At the surface of the oceans where the water becomes warmer, dissolved carbon dioxide is released back into the atmosphere.
vi) Volcanic eruptions release CO2 into the atmosphere, in addition to other gases.